Promoters are regulatory elements that direct the expression of genes. Both constitutive and regulated promoters are used to direct gene expression in transgenic organisms including plants. Constitutive promoters direct expression in most or all tissues, and are useful when high levels of production of a gene product are desired. The 35S promoter from cauliflower mosaic virus (CMV) is frequently used to direct constitutive expression. Regulated promoters, such as tissue-specific and inducible promoters, are used to direct spatially or temporally specific expression, or expression in response to environmental factors.
The present invention is directed to a promoter that directs constitutive expression of genes in plants. The present promoter is derived from an ATP sulfurase gene of Arabidopsis thaliana. 
ATP sulfurase catalyzes the formation of adenosine-5′-phosphosulfate from ATP and sulfate in the sulfate assimilation pathway. In Arabidopsis, ATP sulfurase is encoded by a gene family of at least three genes, APS1, APS2 and APS3. Leustek et al. (1994) Plant Physiol. 105:897; Murillo et al. (1995) Arch. Biochem. Biophys. 323:195; Klonus et al. (1995) Plant Physiol. 107:653; Logan et al. (1996) J. Biol. Chem. 271:12227. APS1, APS2 and APS3 cDNAs have been isolated and sequenced. Leustek et al. (1994); Murillo et al. (1995).
In accordance with the present invention, it has been discovered that a promoter derived from APS1 can direct high levels of constitutive expression of heterologous genes in plants.